1. Field of the Invention
The present invention relates generally to wireless networks. More particularly, the invention relates to a medium access technique for wireless networks.
2. Description of the Related Art
Conventional wireless medium access solutions generally fall into two categories: contention-based and scheduled (also termed contention-free or non-contention based). It has been well established that the collision-free approach of scheduled schemes provides high throughput in high demand scenarios at the expense of overhead and packet delay. In comparison, contention-based approaches provide low delay times at low to moderate network loads, but performance begins to degrade rapidly as the load increases and the network becomes saturated.
Although initial work has been done in the wireless sensor network field to combine the benefits of both approaches in response to changing network load, no medium access scheme has been proposed that is capable of accommodating effectively and efficiently the multiple flows that exist in a wireless sensor network. While overall network performance may be optimized, individual flows may perform poorly in existing wireless sensor network medium access control solutions.
Carrier sense multiple access (CSMA) [KLE75] and its many variants are among some of the most widely studied proposals to provide contention-based medium access to wireless networks. They feature good throughput and delay performance at low to medium loads, but performance falls off rapidly as the load increases. Furthermore, the protocols are very energy-intensive and require nodes to constantly monitor the channel. The IEEE 802.11 standard [802.11] is built upon the slotted, non-persistent CMSA protocol and reflects these benefits and shortcomings.
The IEEE 802.15.4 standard [802.15.4] and Sensor-MAC (S-MAC) [YE04] are the most widely studied among a large group of contention-based medium access schemes that have been proposed for wireless sensor networks. IEEE 802.15.4 uses both slotted and unslotted CSMA and features a sleep cycle that improves energy efficiency. A contention-based solution, 802.15.4 reflects the same performance trends common to other CSMA-based approaches. Furthermore, it relies on a centralized controller to implement the sleep cycle. S-MAC provides a distributed solution to sleep cycle coordination, but still suffers from the fall off in performance at heavy loads.
Time division multiple access (TDMA) [LAM77] is a common contention-free approach to medium access. It performs well at high loads, but suffers from increased overhead and inefficient operation at low to medium loads. The Traffic-adaptive Medium Access (TRAMA) protocol [RAJ06] is a well-studied TDMA-based solution for wireless sensor networks that allows nodes to sleep through the use of a preschedule scheme but suffers from the inefficient operation at low to medium loads.
Finally, Z-MAC [RHE05] is the most well known of several hybrid approaches that have been proposed in the literature for wireless sensor networks. These hybrid solutions attempt to combine the performance of contention-based medium access approaches at low-to-moderate loads and non-contention-based approaches at high loads. Z-MAC uses a CSMA-based medium access scheme in the contention mode and a TDMA-based scheme in the non-contention mode. Energy savings are achieved utilizing a sleep schedule that is coordinated through preamble sampling. The salient shortfall in these hybrid approaches is that they treat the flows in aggregation rather than individually.